The present invention relates to a heating conductor, in particular for a sensor, and a method for manufacturing the heating conductor.
Heating conductors of the type mentioned are known and are used for setting an adjustable operating temperature of the sensor. Sensors of this type are marked by an advantageously layered design, individual layers being obtained using silk-screen printing, laminating, stamping, sintering, or the like. If the sensor acts to determine an oxygen concentration from the exhaust gases of an internal combustion engine, it contains essentially the following features:
A measuring electrode is arranged on a surface of the sensor and, if appropriate, is covered by a porous protective layer. Underneath the measuring electrode is located a layer composed of a solid electrolyte, that layer being followed by a reference electrode. The reference electrode in turn is situated on a reference gas channel, which is filled with a reference gas. To bring the sensor element to a specifiable temperature, there is arranged underneath the reference gas channel the heating conductor, which is optionally covered by an electrical insulator. The electrodes and the heating conductor are usually manufactured by sintering a mixture made of a metal oxide powder and a metal powder (cermet).
In manufacturing a sensor of this type, it is necessary, for one thing, to ensure that the measuring and reference electrodes have sufficient porosity, becausexe2x80x94without going into greater detail here concerning a mode of functioning of a sensor of this typexe2x80x94a sufficiently large 3-phase boundary surface must be provided for assuring functionality. In order to prevent the measuring and reference electrodes from becoming densely sintered (no porosity), correspondingly low sintering temperatures must therefore be selected.
For another thing, the heating conductor must have a sufficient current carrying capacity, which is all the more beneficial, the lower the porosity of the heating conductor. Therefore, for manufacturing the heating conductor, the highest possible sintering temperature is preferred.
Known heating conductors are usually made of a cermet composed of platinum and a metallic oxide, such as aluminum oxide. From U.S. Pat. No. 5,787,866, it is known to manufacture the heating conductor out of platinum Pt and a further precious metal from the group Rh, Pd, Ir, Ru, and Os, in order to increase its resistance to corrosive processes. Since the measuring and reference electrodes preferably also contain platinum as a metallic component, the sintering temperature can only be selected as a compromise between the two objectives with respect to porosity.
Furthermore, it is disadvantageous that the known heating conductors have only insufficient operating stability on account of the oxidation of the platinum and the coagulation of the Pt. Because of aging processes of this type, the sensor can be subject to a total failure.
It is also disadvantageous that in manufacturing the known heating conductors using sintering, it is difficult to control the influence on the heating conductor resistance of unavoidable temperature differences in the sintering furnace as well as of the duration of a temperature treatment.
The aforementioned disadvantages are eliminated using a heating conductor manufactured with a cermet that has added to it at least two further precious metals, thereby allowing a lower sintering temperature. A lower sintering temperature creates a heating conductor with a low porosity, and, reduces the influence of temperature fluctuations in the sintering furnace. Furthermore, a heating conductor of this type demonstrates a significantly lower susceptibility to oxidation, so that the heating conductor has an increased service life.
It has proven to be particularly advantageous to select the further precious metals from the group Pd, Rh, Au, Ag, and Ir. In this context, the cermet should have the composition
(a) 0.5 to 50% wt metal oxide,
(b) 35 to 95% wt platinum,
(c) 0.5 to 30% wt of the further precious metals,
the cited quantities referring to the overall quantity of components (a), (b), and (c). In particular, a composition of the cermet of 6.6% wt Rh and 3.3% wt Au, as well as either 88.1 % wt Pt and 2% wt Al203 or 80.7% wt Pt and 9.4% wt Al203, have proven to be especially advantageous with respect to the resistance and the manufacture of the heating conductor.
As a function of the embodiment of the sintering method and/or of a property of the metals used for manufacturing the cermet, it is possible to influence the distribution of the metal components in the cermet. Thus it is conceivable that cermets in which heterogeneous alloys of platinum and the further precious metals are present are manufactured in a controlled manner. In this manner, the resistance of the heating conductor can additionally be influenced.
In addition, the method steps necessary in manufacturing the heating conductor with respect to the use of platinum-precious metal alloys can be developed in an advantageous manner. Thus, during the sintering, precious metals can precipitate out in the area of the measuring and reference electrodes due to their high vapor pressure. As a result of an often high CO affinity of the precious metals, this can lead to a falsification of the measuring value of the sensor. This can be prevented during the sintering in the sintering furnace if a sufficiently large air exchange is assured, for example using a blower. In this context, it is advantageous to introduce in the sintering furnace condensation areas for the precious metal.
Another method provides that the sintering furnace be designed such that a temperature gradient is present within the sensor element in the furnace. In this context, the sensor is arranged in the sintering furnace such that the heating conductor is located in an area having the lowest temperature, so that the precious metal only condenses in this area.